1. Field of the Invention
The present invention relates to an apparatus for transmitting an optical signal by a wavelength division multiplexing using multiple wavelength light supplied from a multiple wavelength light source.
2. Description of the Related Art
Today, a communication capacity of optical communication has seen a quantum leap of increase with the commercialization of a wavelength division multiplexing, WDM) communication technique (e.g., refer to patent documents 1 through 9 listed below). With a movement of optical fibers migrating to all the transmission paths in the client systems, a further increase of communication capacities is in strong demand.
Patent document 1: Japanese patent laid-open application publication No. 2001-197006
Patent document 2: Japanese patent laid-open application publication No. 11-261532
Patent document 3: Japanese patent laid-open application publication No. 04-336829
Patent document 4: Japanese patent laid-open application publication No. 07-177556
Patent document 5: Japanese patent laid-open application publication No. 2000-277849
Patent document 6: Japanese patent laid-open application publication No. 2003-188821
Patent document 7: Japanese patent laid-open application publication No. 11-127136
Patent document 8: Japanese patent laid-open application publication No. 2000-183817
Patent document 9: Japanese patent laid-open application publication No. 08-023308
FIG. 1A shows a configuration of such a WDM transmission system. The WDM transmission system shown by FIG. 1A includes a terminal station A, a relay station B and a terminal station C. The terminal station A comprises transmission units 11-1 through 11-5, receiving units 12-1 through 12-5, and a wavelength multiplexing & separation apparatus 13-1; and the terminal station C comprises transmission units 11-16 through 11-20, receiving units 12-16 through 12-20 and a wavelength multiplexing & separation apparatus 13-4. The relay station B comprises transmission units 11-6 through 11-15, receiving units 12-6 through 12-15, wavelength multiplexing & separation apparatuses 13-2 and 13-3, and an electric ADD & DROP apparatus 14.
Each of the transmission units 11-1 through 11-20, including a light source 21 with a certain wavelength and a modulator 22, modulates light from the light source 21 by a transmission data string to generate an optical signal as shown by FIG. 1B. Each of the wavelength multiplexing & separation apparatuses 13-1 through 13-4 includes a wavelength multiplex unit 15, a wavelength separation unit 16, an optical transmission amplification unit 17 and an optical receiving amplification unit 18.
The optical signals of respective wavelengths outputted from the transmission units 11-1 through 11-5 comprised by the terminal station A are multiplexed by the wavelength multiplexing & separation apparatus 13-1 and transmitted to the relay station B as WDM light. In the relay station B, the received WDM light is separated into optical signals with respective wavelengths by the wavelength multiplexing & separation apparatuses 13-1 so as to be converted to electrical signals by the receiving units 12-1 through 12-5. The electric ADD & DROP apparatus 14 branches (i.e., drops) a part of the received signal or inserts (i.e., adds) another transmission data string thereto.
Then, a WDM light is transmitted from the relay station B to the terminal station C in the same way as the transmission from the terminal station A to the relay station B, and the optical signals of the respective wavelengths are received by the receiving units 12-16 through 12-20 therein. The procedure for the transmission from the terminal station C to the terminal station A is the same as that from the terminal station A to the terminal station C.
In such a WDM transmission system, increasing the number of wavelengths in order to increase the communication capacity of the system is relatively simple. More and more increase in wavelength band, however, makes a transmission impossible by limitations such as light amplification band, transmission band of optical fiber, bandwidths of optical devices, et cetera. This makes it necessary to increase the number of wavelengths by narrowing a wavelength interval instead of increasing the wavelength band which is limited to the most effective width.
A gain wavelength band of a commonly used multi-wavelength EDFA (erbium doped fiber amplifier) which is equipped in the optical transmission amplification unit 17 and optical receiving amplification unit 18 for each band such as L-band, C-band and S-band is approximately between 28 and 32 nm. Therefore, the number of wavelength multiplexing varies with how many wavelengths are packed within the range of the gain wavelength band as shown by FIG. 1C.
In this event, a precision of light source for the each wavelength becomes an issue as a factor to prevent an increase in the number of wavelengths. If optical signals are generated by installing a light source for each wavelength independently in the applicable transmission unit as shown by FIGS. 1A and 1B, an error Δλcont in the autonomous oscillation accuracy of each wavelength occurs as shown by FIG. 1D.
Meanwhile, a pass characteristic of an optical device (i.e., wavelength filter) such as an arrayed waveguide grating (AWG) which is used as the wavelength multiplex unit 15 and wavelength separation unit 16 will of course encounter a variation in its production.
For instance, the pass characteristic in the case of WDM light coming into a port P3 of a wavelength filter shown by FIG. 1E and optical signals of wavelengths λ1 and λ2 being outputted from ports P1 and P2, respectively, is as shown by FIG. 1F. In FIG. 1F, a curve 31 indicates an optical loss from the port P3 to port P1, while a curve 32 indicates that from the port P3 to port P2. In order to separate these optical signals by using a wavelength filter, the distance between the λ1 and λ2 needs to be no less than Δλfilter, taking the production variation into consideration.
Moreover, assuming that the spectrum of light expands in a modulation by Δλmod., the distance of wavelength Δλ between the λ1 and λ2 needs to comply with the condition as follows:
Δλ>Δλcont+Δλfilter+Δλmod.
As described above, the method of narrowing the distance between wavelengths is understandably limited when considering the factors such as a wavelength accuracy of light source, a production variance of wavelength filter, et cetera. In the meantime, being investigated is a method for increasing the number of wavelengths without narrowing the distance between wavelengths by using a Raman amplification technique for widening an optical amplification bandwidth.
Besides, an increase in the number of wavelengths will require the same number of laser oscillators emitting in a precisely different wavelength and a suitable wavelength interval, resulting in the cost of the part thereof occupying the majority of that of the system.
Accordingly, a cost reduction by revisiting a configuration of light source becomes effective in an attempt to assist a quantum leap of communications capacity. One of such methods being considered is the one for a multiple wavelength light source supplying multiple wavelength light to a plurality of stations.
FIG. 1G is a configuration of WDM transmission system by using such a multiple wavelength light source. The WDM transmission system shown by FIG. 1G, vis-à-vis the configuration shown by FIG. 1A, replaces the transmission units 11-1 through 11-20 with transmission units 42-1 through 42-20; adds a wavelength separator 41-1 to the station A, wavelength separators 41-2 and 41-3 to the station B, and a wavelength separator 41-4 to the station C; adds an optical coupler 43 to the station B; and further adds a station D.
Each of the transmission units 42-1 through 42-20, being configured by removing a light source 21, vis-à-vis the configuration shown by FIG. 1B, modulates externally inputted light by a transmission data string to generate an optical signal as shown by FIG. 1H. The station D, comprising a multiple wavelength light source supply apparatus 44, supplies CW (continuous wave) light (i.e., multiple wavelength light) containing light of multiple wavelengths to the stations A through C. The optical coupler 43 added to the station B branches the supplied multiple wavelength light into two to supply the wavelength separator 41-1 with the one and the wavelength separator 41-3 with the other.
In the station A, the wavelength separator 41-1 separates the supplied multiple wavelength light into lights of the respective wavelengths to supply the transmission units 42-1 through 42-5. Each of the wavelength separator 41-2 through 41-4 added to the stations B and C likewise fills the roles of separating the multiple wavelength light supplied by the multiple wavelength light source supply apparatus 44 into lights of the respective wavelengths.
Multiple wavelength light generated by one multiple wavelength light source is capable of retaining intervals among the wavelengths even after passing through the wavelength separator 41-2 through 41-4. Therefore, there is no longer need to concern with the above described error Δλcont in the oscillation accuracy. And there is no need to equip a laser oscillator with every transmission unit, hence enabling a reduced cost for the light source part as an overall system.
Meanwhile, the recent years have seen a commercialization of a photonic crystal fiber, PCF, which is suitable to a multiple wavelength simultaneous transmission and a development of multiple wavelength batch conversion technique such as one being represented by a periodically poled lithium niobate, PPLN, as a multiple wavelength conversion element. A method for utilizing these new techniques is in undeveloped regions and a future market expansion is expected.
The above described WDM transmission system by using a multiple wavelength light source, however, has been faced with the problem as follows. The system shown by FIG. 1G needs to add more optical fibers by the number of stations for supplying multiple wavelength light to each station, hence magnifying the cost therefor. Also, if a supply of light from the multiple wavelength light source is interrupted, then the station is cut off from all the communications, since the light source is not installed in each of the stations.